The story is related of Hulegu, the Mongol conqueror of Baghdad in 1258, that he once encountered a band of Qalandar sufis. Those were the wild and woolly ones, who deliberately flouted social conventions and even the sharia in various ways to show how holy they really were. Hulegu asked the Muslim leader Nasir al-Din Tusi, who was accompanying him, who these social misfits were, and Tusi replied that they were the excess waste of society. So Hulegu ordered them all killed. I am not a Mongol general, but after a frustrating series of miscalculations I can harbor similar feelings as Tusi to the plethora of calendars in the Middle East.
Most societies have found it useful to have a common calendar, and many indeed come to regard their shared ways of marking time as so natural that they forget the existence of alternatives. Years, months, and days are simply natural phenomena with uncomplicated definitions. Until one encounters a radically different calendar.
One of the first lessons all historians of Islam need to learn is how to reckon with the Hijri calendar. This reckons years since the emigration (hijra) of Muhammad from Mecca to Medina in the middle of his prophetic career, according to the traditional narrative of Muslim origins. That event is conventionally dated to 622 CE, but the current Hijri year is 1436 instead of 1393 because each Hijri year is roughly 11 days shorter than each Gregorian year. This is not arbitrary: the Gregorian calendar is strictly solar, while the Hijri calendar is entirely lunar. Gregorian months are each defined to be a specific number of days, more or less 1/12 of the days in the period of the sun’s observed variation from winter solstice to winter solstice. Hijri months, on the other hand, are based precisely on the phases of the moon, and after twelve new moons, a year has passed, regardless of the observed position of the sun. In many locales, the declaration of a new month actually required an observation of a new crescent moon, which could be delayed by poor visibility conditions, resulting in months being a day or two off in different areas using the same calendar. But astronomy improved to be able to predict new moons, and so now most areas using the Hijri calendar do so on the basis of computation. Fortunately for scholars, there are now online calendar converters (such as here) which can compute the equivalences for you, rather than needing to do so by hand. Islamicists get accustomed to giving two dates, the Hijri date followed by the Western (Julian or Gregorian) date, separated by a slash: e.g. 21 Rabi’ al-Awwal 1436 / 12 January 2015.
Yet working in two radically unrelated calendars is not nearly complicated enough to deal with the actual history of the Middle East. Although the lunar Hijri calendar is called the Islamic calendar, in fact other calendars have been used by Muslims. For around a century Iran and Afghanistan have used a solar calendar counting years since Muhammad’s hijra, so the year is 1393. A medieval predecessor was adopted in 1079 CE. In 1840, the Ottoman Empire adopted a solar calendar starting on March 1, with the current year number being the same as that of the Hijri calendar at that time. In other words, this Rumi calendar measured solar years from a non-event some 38 years before the hijra. The month names for this solar Rumi calendar were entirely taken from Greek or Syriac months. The use of Christian month names was also seen in Egypt from at least the Mamluk period onward. Because the Egyptian economy depended so heavily on the seasonal flooding of the Nile, and the seasons are tied to solar rather than lunar cycles, medieval Muslim authors found it useful to refer to the passing of the Coptic (solar) months.
But the partial adoption of solar calendars by Muslims in different periods hints toward a greater number of Middle Eastern calendars used by non-Muslims. Maimonides codified the rules of the earlier Jewish ritual calendar in 12th C Egypt, which is based on lunar months, but adding a 13th month every few years to keep the months approximately aligned with the seasons. In this mixed luni-solar calendar, the years count from when the creation of the world was supposed to have happened. The Coptic Christians of Egypt and the Syriac Christians of Syria, Jazira, and Iraq used solar calendars which kept pace with the Julian calendar, although the former considered 1 September as the new year, while the latter started the new year with 1 October. The Copts counted their years since the persecution of Diocletian under Roman rule, while the Syriacs counted from the accession of Seleucus I Nikator to the throne in Babylon following the death of Alexander the Great. (In the medieval period, Syriac Christians in Iraq mistakenly believed they were keeping track of the years from the beginning of Alexander’s reign.) The Julian calendar itself was used by Latin pilgrims and crusaders in the Middle East, while the Armenians rejected leap years and simply had 365 days in every year, meaning that the Armenian calendar shifted one day forward relative to the Julian calendar in every four years. The result is that there is an extra Armenian year every 1460 Julian years, and the Armenian new year moves relative to the seasons, but far more slowly than the Hijri calendar does. The Armenian calendar counts from 551 CE, which means they just earned their extra year in 2011. But a Middle Eastern Christian leader was on the papal commission in the 1570s which recommended the calendrical reform leading to what we now know as the Gregorian calendar.
This calendrical pluralism was well-known in the medieval Middle East. Astronomical manuals, such as the zij of Ulugh Bey, the Timurid ruler of Samarqand in the mid-fifteenth century, enumerated the different calendars, how to compute them and convert between them, and what notable days each had. The Middle East has not been calendrically unified at any point in the past two millennia, a state of affairs which continues today with the widespread employment of the Gregorian calendar for international uses and the Hijri calendar for religious purposes. In the meantime, the Christian populations of Egypt and Iraq still use their calendars equivalent to the Julian calendar for their religious observances, which means that they celebrate Christmas on January 7 (Gregorian) and Epiphany on January 19 (Gregorian). And since “the Christmas season” for them follows rather than precedes Christmas itself, the surfeit of Middle Eastern calendars explains why Cairo and Baghdad are still in the Christmas season in the middle of January.